Encyclopedia entry
How does semaglutide work?
Oliver Mackman · Editorial director · Best Business Loans Ltd (16833937)
Last updated 2026-06-10
Semaglutide is a once-weekly GLP-1 receptor agonist, a medicine that mimics a natural gut hormone called GLP-1. It works in three main ways: it slows how quickly the stomach empties, it acts on appetite centres in the brain so you feel full sooner and stay full longer, and it helps the pancreas release insulin when blood sugar is high. In weight-management trials, average weight loss was around 15 percent of body weight over about 68 weeks. Semaglutide is the active ingredient in Wegovy (weight management) and Ozempic (type 2 diabetes). It is a prescription-only medicine in the UK. This page is editorial commentary, not clinical advice.
The mechanism in plain English
- · Slower stomach emptying. Food stays in the stomach longer, so you feel full for longer after a meal.
- · Appetite signalling. Semaglutide acts on receptors in the hypothalamus, the brain's appetite control centre, reducing hunger and food preoccupation.
- · Glucose-dependent insulin. It prompts the pancreas to release insulin when blood sugar is high, and lowers glucagon, which is why the same molecule is also used to treat type 2 diabetes.
The net effect is that most people eat less without the constant hunger that usually undermines dieting. It does not burn fat directly; it changes appetite and fullness so a calorie deficit becomes easier to sustain. For the related dual-hormone medicine, see how tirzepatide works and GLP-1 vs GIP.
Does it work? What the trials show
In the STEP programme, semaglutide 2.4 mg produced average weight loss of around 15 percent of body weight at 68 weeks, alongside diet and activity changes. That is well beyond what diet and exercise alone typically achieve. Results vary between individuals, and the effect depends on staying on treatment: most people regain a large share of the weight after stopping, which is why aftercare and maintenance matter. For a fuller picture of what is realistic, see hype vs reality.
Is it safe? The UK framing
Semaglutide is a prescription-only medicine, which means a UK-registered prescriber assesses whether it is appropriate and safe for you against your medical history. The most common side effects are gastrointestinal, such as nausea, especially while the dose is increasing. It carries a Black Triangle, meaning the MHRA is monitoring it closely and wants suspected side effects reported through the Yellow Card scheme. The authoritative document for side effects and contraindications is the Patient Information Leaflet in the box. We do not provide medical triage; questions about whether it is safe for you are for your prescriber.
Frequently asked questions
Is Wegovy the same as Ozempic?
Both contain the same active ingredient, semaglutide, but they are different brands licensed for different uses: Wegovy for weight management and Ozempic for type 2 diabetes. See our brand confusion explainer.
How long does semaglutide take to work?
Appetite changes are often noticed within the first few weeks, but the dose is increased gradually over months and the trial results reflect roughly 68 weeks of treatment. It is not an overnight effect.
Does semaglutide burn fat directly?
No. It reduces hunger and increases fullness, which makes eating less easier to sustain. The weight loss comes from the resulting calorie deficit, not from the drug burning fat.
Is semaglutide a peptide?
Yes. Semaglutide is a peptide medicine, a modified version of the natural GLP-1 hormone engineered to last about a week in the body. Unlike research peptides, it is fully MHRA-licensed.
Related: how tirzepatide works · semaglutide vs tirzepatide licensing · Wegovy profile.